1. Field of the Invention
The present invention relates to a linear measuring system including a modular scale having support means and a plurality of graduation carriers mounted on the support means and arranged one behind another in a longitudinal direction, with the support means and the graduation carriers having substantially same coefficients of thermal expansion; and a measuring head for scanning the scale.
The present invention also relates to a method of manufacturing such a linear measuring system.
2. Description of the Prior Art
German Patent No. 1,773,460 disclose a particularly long modular scale formed of several shorter scales. The drawback of this modular scale consists in that at butted locations of the shorter scales, which are arranged in a row one after another, the gap between the adjacent shorter scales cannot be precisely determined and, therefore, the results of the measurement are imprecise. To eliminate this imprecision, two scanning heads for scanning the scale are provided, which are fixedly connected with each other with a predetermined distance therebetween. Further, there is provided switch means which are controlled by evaluation electronic means and with which scanning with one head is switched to scanning with another head when the scanning unit approaches a butt joint. The switching is effected with actuation means provided on both sides of the butt joint of two scales which actuate the switch means. The switching is affected in such a way that scanning with a head, which finds itself a short distance in front of the butt joint in the movement direction of the scanning unit, is switched to scanning with a head which is located a short distance behind the first head. The gap width is determined by the evaluation electronic means in accordance with the switching data.
The drawback of this solution consists in that additional switching and actuation means need be provided. Further, two scanning heads with predetermined distance there-between need also be provided. Still further, even with the use of these additional means, the measuring results can still be imprecise, in particular with small graduation periods of the graduation. The imprecision is caused by mechanical switching which, because of scattering of the switching time points and the use of actuation means, is not precise.
German Patent No. 2,730,715 likewise discloses a linear encoder with a scale formed of several shorter scales. As in German Patent No. 1,773,460 discussed above, there is provided a scanning unit with two scanning heads spaced from each other by a predetermined distance, with switching of scanning from one head located closer to a butt joint to another head located behind the first head, and with evaluation of the switching results by an electronic unit. The linear encoder of German Patent No. 2,730,715 differs from that of German Patent No. 1,773,460 in that the switching is effected not with mechanical means but with electronic means comprising a synchronization track provided on the scale, which track is scanned with special scanning means in order to detect the switching time points.
This linear encoder has substantially the same drawbacks as the linear encoder of German Patent No. 1,773,460, with the only difference that instead of mechanical switching means, actuation means of German Patent No. 1,773,460, electronic switching means in the form of a synchronization track and special scanning means are used.
A linear encoder with a scale formed of a plurality of shorter scales is also disclosed in German document No. 3,818,044. In the linear encoder of this document, the butt joint of the scale are defined by special coded fields the length of which is larger than the length of a conventionally used coded field. When such longer coded field is detected, it is evaluated. In this linear encoder, also two scanning heads are provided with switching from one head to another at a butt joint of two adjacent shorter scales to obtain necessary data.
The drawback of this linear encoder consists in that additional coded fields having an increased length need be used to enable a detection of a butt joint, and in that at least two scanning heads, switching means and evaluation electronics are required.
German Patent No. 1,548,875 discloses a linear encoder in which the scale is formed of a plurality of foil elements provided with graduation, which are glued in a groove formed in a support, with the longitudinal sides of the foil elements being welded to the support. This should insure that a long lasting, constant and precise gap is retained at the butt joint which is equal to an integral multiple of a graduation period.
The drawback of this linear encoder consists in that the heat generated at the welding points deforms the foil elements. Further, imbedding of the foil elements into a groove, which is formed in the support, complicates the welding process because when the welding energy is fed perpendicular to the supports, either the foil elements should be melted through their entire thickness or the support should be melted along an entire height of the groove at a predetermined region. If the welding energy is not fed perpendicular to the support, the welding process becomes very complicated and expensive. A further drawback consist in that with the edge of the graduation carrier being welded with the edge of a support groove, a relatively large amount of the welding energy is carried away from the support. Therefore, in order to obtain a desired welding connection, a relatively large amount of welding energy need be supplied, which results in the deformation of the graduation carrier which can make it unusable.
German document No. 2,727,769 discloses a sealed linear encoder in which the graduation carrier, the support and the hollow section, which insures sealing of the linear encoder, are formed of a plurality of separate sections arranged in a row one after another. At that, the graduation carrier is fixed to the support, with the support being floatingly arranged in the hollow section. To provide a gap width equal to a integral multiple of a graduation period between two adjacent sections, corresponding adjusting screws are provided in the support. The separate sections are connected with each other with a certain pre-load which is insured by spring means.
The drawback of this encoder consists in that the use of adjusting screws is required in order to provide a gap of a predetermined width equal to an integral multiple of a graduation period at a butt location. A further drawback consists in that the adjustment of the gap width is effected only during the assembly of separate sections with each other. That means that the adjusting cannot be effected during manufacturing.
German document No. 2,505,587 discloses a linear encoder in which the graduation is provided on a single surface of a bending-resistant hollow body with which the linear encoder is sealed. The graduation is secured to the hollow body with a highly elastic, thin, intermediate layer. Thereby, generation of constraining forces is prevented, which may result from different coefficients of thermal expansion of different elements of the encoder.
German document No. 3,302,643 discloses a clamping device for a linear measuring system with a measuring standard carrier being secured to the bearing surface of a measured object with at least one clamp which engages in a side groove provided in the carrier. The clamp has three projection one of which engages in the groove, the other of which is supported on the bearing surface, and the third one serves for supporting the carrier. At that, during the attachment of the clamp, a slight tipping of the clamp is possible.
The drawback of this system consists in that the bearing surface of the object has to be of a high quality, and that the entire bottom surface of the carrier should abut the object, with the both bottom surface of the carrier and the clamp support surface of the carrier being also of high quality.
Accordingly, an object of the present invention is a linear measuring system having a modular scale and a method of producing such a system both of which are simple and cost-effective.
Another object of the present invention is a linear measuring system and a method of producing the same which would insure obtaining error-free results with the graduation having a small graduation period.